1. Field of the Invention
The present invention relates to a mold for nano-imprinting, a method for forming a diffraction grating, and a method for producing an optical device including a diffraction grating.
2. Description of the Related Art
Instead of a known photolithography method, a nano-imprint lithography (NIL) method has been proposed to form a semiconductor optical device. In such a nano-imprint lithography method, a mold is prepared. The mold has a pattern portion in which a predetermined pattern having projections and recesses is formed. The mold is then pressed against a resin layer formed on a substrate. Thus, the pattern formed on the mold is transferred to the resin layer. The substrate is etched using, as a mask, the resin layer in which the pattern having projections and recesses is formed, and consequently the pattern having projections and recesses is formed on the substrate. In this method, when the projection of the pattern has a height comparable to a surface roughness of the substrate, part of the pattern portion of the mold is not pressed against the resin layer formed on the substrate. In other words, the pattern having projections and recesses of the mold is not uniformly transferred over the entire resin layer. As a result, the pattern having projections and recesses is not uniformly formed on the substrate.
Japanese Unexamined Patent Application Publication No. 2010-36514 discloses that nano-imprinting is conducted using a mold made of a flexible material. Since the mold deforms along the surface of a substrate even when the surface of the substrate is not flat, the pattern portion is fully pressed against the entire resin layer on the substrate. As a result, by etching the substrate using the resin layer as a mask, the pattern having projections and recesses can be formed over the entire substrate.